1. Field of the Invention
The present invention relates to the structure of a transmission filter and reception filter of an antenna sharing device.
2. Description of the Related Art
Mobile radio apparatuses, such as cordless main and sub-telephones and mobile telephones, communicate bi-directionally using a single antenna shared for transmission and reception.
Such an antenna uses an antenna sharing device for separating transmission and reception signals so as to prevent them from being mixed with each other at a transmitter or receiver.
FIG. 13 is a block diagram of an antenna sharing device. An antenna 1 is coupled at an antenna branch node 2 to a transmission filter 5 and to a reception filter 6. The transmission filter 5 is connected to a transmitter (not shown) at a terminal 3, and the reception filter 6 is connected to a receiver (not shown) at a terminal 4. Transmission and reception signals are separated by the filters 5 and 6.
FIG. 14 is a circuit diagram showing part of a transmission filter of a conventional antenna sharing device. This transmission filter is constructed of a band-pass filter and a parallel resonance circuit 11. The parallel resonance circuit 11 is connected between an antenna branch node 2 and the output terminal of the band-pass filter which is constructed of a plurality of parallel resonance circuits 10 and coupling capacitors C.sub.9. The input terminal of the band-pass filter being connected to a transmitter. The band-pass filter passes only transmission signals and intercepts unnecessary signals, and the parallel resonance circuit 11 prevents reception signals from entering the transmitter.
Such a transmission filter requires a number of coils of the parallel resonance circuits 10 and 11. Moreover, each parallel resonance circuit is required to be shielded within a case, making its size large and resulting in a large size antenna sharing device.